Separate absorption and detection diode for two-color operation

ABSTRACT

A photodiode for detection of preferably infrared radiation capable of detecting two different wavelengths wherein photons are absorbed in one region and detected in another. In one example embodiment, an absorbing P region is abutted with an N region of lower doping such that the depletion region is substantially (preferably completely) confined to the N region. The N region is also chosen with a larger bandgap than the P region, with compositional grading of a region of the N region near the P region. This compositional grading mitigates the potential barrier between the respective bandgaps. Under first voltage conditions a potential barrier prevents minority carriers from moving from the P region to the N region, but photons of energy large enough to generate minority carriers within the N region are detected. Under reverse bias, the barrier is substantially reduced or disappears, allowing charge carriers to move from the absorbing P region into the N region (and beyond) where they are detected. Thus, under reverse bias, both wavelengths of energy can be detected, and separated by differencing the output.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional patentapplication 60/718,840 filed on Sep. 20, 2005, which is herebyincorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

1. Field of the Invention

The present inventions relate generally to detection of radiation, suchas infra-red radiation.

2. Description of Background Art

A photodiode is an electrical component that behaves as a photodetector.Photodiodes are typically implemented as p-n junction diodes that areresponsive to optical input, for example, by providing a window, fiber,or other means for photons to impinge on a light-sensitive part of thedevice.

Many types of photodiodes operate in reverse bias mode. Diodes typicallyhave high impedance when reverse biased, and light of a proper frequencycan generate charge carriers to move from the valence band into theconduction band, and are detected in the high impedance diode circuit,thereby allowing sensitive measurement of the light.

FIG. 1A depicts an energy band diagram of a basic p-n junctionphotodiode. In this diagram, electron energy is plotted vertically,while spatial distance is plotted horizontally. The diagram includesconduction band 102, valence band 104, and Fermi energy level 106. Thedistance between valence band 104 and conduction band 102 is called theenergy gap, or bandgap, 108. In this example, p-type semiconductor 110is on the right, while n-type semiconductor 112 is on the left. Junctionregion or depletion region 114 is shown near where the p-type region 110and the n-type region 112 meet. An electric field E 116 exists in thisregion. The field 116 exerts a force on charge carriers, moving holes tothe right and electrons to the left. This field is a result of theexposed charge in the depletion region, where mobile carriers aredepleted by action of the field.

FIG. 1B shows a p-n junction photodiode under reverse bias, a typicaloperating condition. As in FIG. 1A, the electric field 116 is a resultof the formation of a junction or depletion region between 114 the twosides. If a photon 118 with energy greater than bandgap energy 108strikes regions 110, 112, or 114, electron and hole pairs may begenerated by absorption of the photon, which excites an electron fromthe valence band 104 to the conduction band 102. In typical p-n junctionphotodiodes, region 114 with electric field 116 is considered the photonabsorption region of the photodiode, together with those parts ofregions 112 and 110 within a minority carrier diffusion length of region114. The two outer regions 112, 110, are considered contact regions forcollecting photogenerated carriers. Depletion region 114 combined withminority carrier diffusion length regions of 112 and 110 constitute thephoton absorption region.

Because the same material is used throughout the device in this example,the bandgap has a constant value across the junction. Such a junction isknown as a homojunction, because the junction between semiconductorsdiffers only in doping levels, and not in alloy or atomic composition.

FIG. 1C shows a similar p-n junction diode, this one comprising aheterojunction wherein the bandgap between p-type and n-type regions donot match. In this example, the valence bands 104 of the two halvesmatch, but the conduction bands 102 show a discontinuity 120 where thedifferent bandgap materials meet. The bandgap 108A of the p-type region110 is smaller than the bandgap 108B of the n-type region 112. Thediscontinuity 120, in this example, provides a potential barrier againstminority carrier flow from the p-type region to the n-type region, evenunder reverse bias. In this Figure the band-bending is explicitly shown,unlike in the band diagrams of FIGS. 1A and 1B, and hence the valenceand conduction band levels are slightly curved at transition points.

Photodiodes, like other electrical devices, experience noise on thecurrent signal. Noise in a photodetector can arise from a combination ofsources, including Auger, Shockley-Read-Hall, radiative recombination,and background photon flux. When a photodetector has no light input, theoutput of the photodetector is called dark current, which consistsmainly of diffusion currents from either side of the junction, depletioncurrent from S-R centers in the depletion region of the diode, and somecontribution from tunneling currents. For temperatures such thatn_(maj)>n_(i), (where n_(maj) is the majority dopant concentration oneither side of the junction, and n_(i) is the intrinsic carrierconcentration) the overall dark current of a well-made diode isdominated by the S-R centers in the depletion region of the diode.

One driving requirement in photodetectors is to reduce dark current. Theideal photodetector cell should produce an electrical signal whichincreases with increased illumination, but which drops to zero whenthere is no illumination. This is not normally possible in practice.Dark current not only provides a background “noise” signal which makesimages less understandable, but also increases the power consumption ofthe device.

One of the causes of dark current is the aforementionedShockley-Read-Hall (S-R) current from generation/recombination centersin the semiconductor's crystal lattice. Any discontinuity or foreignatom in the crystal lattice can provide a location where carrier pairs(electron+hole) can be generated. These S-R centers generate diffusioncurrents in the electric field-free regions of the diode as well asdepletion generation-recombination (g-r) current in the diode depletionregion.

Another source is Auger generation. A “hot” carrier (electron or hole)is one which has more energy than the minimum for its band. In p-typematerial, if a “hot” hole has more than a bandgap's worth of excessenergy, it can share its excess energy to promote a valence electron tothe conduction band. This electron is a minority carrier in p-typematerial, and will be a component of dark current. In n-type material,if a hot electron has more than a bandgap's worth of excess energy, itcan share its excess energy to similarly generate a mobile hole, whichas a minority carrier can contribute to dark current. (This is a verybrief summary of the “Auger7” and “Auger1” processes, which areextensively described in the technical literature.) An importantdifference between these processes is that the time constant for theAuger7 process (in p-type material) in materials systems with a directenergy gap at k=0 is roughly 10-50 times longer than the time constantfor the Auger1 process (in n-type material). Thus the Auger component ofdark current is much less important in p-type material, if other factorsare comparable.

Reduction in dark current is highly desirable. One way to exploit areduction in dark current is to reduce cooling requirements: infraredimagers are often cooled below room temperature, e.g. with athermoelectric cooler (“cold finger”), or even with cryogenicrefrigeration systems. However, such cooling requirements—especially ifcooling much below 0 C is required—add greatly to system cost, bulk,power consumption, and complexity.

Dark current can be particularly problematic for infrared detectors,since the bandgaps of the absorbing material in such detectors arerelatively small, in order to permit excitation of carriers into theconduction band by absorbing infrared wavelength photons (which are ofrelatively low energy).

At very low temperatures, even the shallowest acceptor dopants (whichionize completely at room temperature) may not be completely ionized.For example, if arsenic is used as the acceptor dopant in HgCdTe, thenat 25K only about one tenth of the arsenic atoms will be ionized. Thatis, instead of an As− ion on a tellurium lattice site (and acorresponding mobile hole), the semiconductor lattice merely contains anunionized arsenic atom on the tellurium site. This is true even if thedopant atoms have been fully activated, i.e. are located on the correctlattice sites and also are not interstitials.

Two-Color Separate Absorption and Detection Diode

The present innovations include a detector system, preferably for twocolors or wavelengths of infra-red electromagnetic radiation, embodiedin a diode architecture. In one example embodiment, the presentinnovations are described as a separate absorption and detection diode,such that photons absorbed in one region of a diode structure (e.g., a Pregion) are detected in another region of the diode structure (e.g., anN region). In an example embodiment, the diode is a heterostructuredevice including a P region which is heavily doped with respect to anadjacent N region, such that the depletion region is substantiallyconfined to the N region of the device. An N+ region is preferablylocated adjacent to the N region. In preferred embodiments, the N and N+regions have wide bandgaps relative to the P region. Further, theinnovative diode structure preferably includes a graded composition inthe N region near the P region such that the resulting field associatedwith the graded bandgap can be overcome by a modest reverse bias voltageapplied to the diode.

In one example embodiment, the present innovations include a diodewherein two different regions have two different bandgaps such that theyrequire different minimum photon energies to excite minority carriersinto the valence and conduction bands. In this example, a secondarycolor (detected in, for example, a wider bandgap region) will bedetected at essentially zero bias across the diode, and a primary color(detected in, for example, a narrower bandgap region) will not registerin the diode output circuit due to the interfacial barrier to minoritycarrier electrons that exists between heterojunction regions of the SADarchitecture. In this example, the primary color signal is read out bythe application of a modest to high reverse bias to the diode. This biasis sufficient to overcome the interfacial barrier, resulting in bothcolors being read out in the output circuit of the SAD diode. Theprimary color is preferably obtained by differencing the output signalsof the SAD diode at low and high reverse biases, though other means ofdistinguishing the signals can also be implemented within the context ofthe present innovations.

The disclosed innovations, in various embodiments, provide one or moreof at least the following advantages:

-   -   wide bandgap nature (of, e.g., the N region) enables a        significant reduction in dark current;    -   graded composition permits conduction band carriers to move from        absorption region to a detection region;    -   wide bandgap nature enables elimination of tunnel currents;    -   p-type nature of absorption region (in preferred embodiments)        enables significant reduction in dark current;    -   wide bandgap nature of diode depletion region enables        significant reduction in dark current;    -   gradient of composition bandgap in transition region is large        enough to eliminate dark current component from the transition        region;    -   barrier to minority carrierflow that exists at low applied bias        enables separation of two colors in readout circuit;    -   the diode is always in reverse bias, thus avoiding any        possibility of IR emission from forward biased regions.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIGS. 1A, 1B, and 1C show energy band diagrams for p-n junctionphotodiodes.

FIG. 2A shows a diagram of a device consistent with implementing apreferred embodiment of the present innovations.

FIG. 2B shows an energy band diagram for a device consistent withimplementing a preferred embodiment of the present innovations.

FIG. 3 shows a diagram depicting various contributions to currents in asemiconductor device.

FIG. 4 shows dark current components for a device where only the dopingconcentrations vary across the junction and material composition (andbandgaps) do not vary.

FIG. 5A shows a diagram of a device consistent with implementing apreferred embodiment of the present innovations.

FIG. 5B shows an energy band diagram for a device consistent withimplementing a preferred embodiment of the present innovations.

FIG. 6A shows a diagram of a device consistent with implementing apreferred embodiment of the present innovations.

FIG. 6B shows an energy band diagram for a device consistent withimplementing a preferred embodiment of the present innovations.

FIG. 7A shows a diagram of a device consistent with implementing apreferred embodiment of the present innovations.

FIG. 7B shows an energy band diagram for a device consistent withimplementing a preferred embodiment of the present innovations.

FIG. 8 shows intrinsic carrier concentration and cutoff wavelengths atvarious operating temperatures for a photodiode.

FIG. 9 shows diagrams of bandgaps consistent with preferred embodimentsof the present innovations

FIG. 10 shows a log plot of tunnel current and bandgap.

FIG. 11 shows various profiles for two different embodiments of thepresent innovations.

FIG. 12 shows a photodiode capable of detecting two differentwavelengths consistent with a preferred embodiment of the presentinnovations.

FIG. 13 shows an array of photodiodes consistent with preferredembodiments of the present innovations.

FIG. 14 shows processing steps for creating a diode consistent withpreferred embodiments of the present innovations.

FIG. 15 shows a timing diagram for two color operation of a photodiodeconsistent with a preferred embodiment of the present innovations.

FIG. 16 shows a charge trans-impedance amplifier input circuitconsistent with implementing a preferred embodiment of the presentinnovations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiment (by way of example, and not of limitation).

In one example embodiment, the present innovations include a separateabsorption and detection (SAD) diode, having an architecture with ap-type infrared (IR) semiconductor absorbing region coupled to anadjacent n-type region of generally higher bandgap than the absorberregion. In preferred embodiments, the p-type region is heavily dopedrelative to the n-region such that the depletion region of the diode issubstantially confined (preferably entirely confined) to the n-type,higher bandgap region of the device. For temperatures such thatN_(maj)>n_(i), where n_(i) is the intrinsic carrier concentration of theabsorbing p-type region, the overall dark current of a homojunctiondiode is dominated by generation through S-R centers in the depletionregion of the diode. Thus, the wide bandgap nature of, for example, then-region (or at least partially graded transition region) of theinnovative SAD diode (depending on the implementation) will enable asignificant reduction in the dark current of the SAD diode, relative tothe corresponding homojunction diode. The bandgap is chosen in then-region to be large enough that the dark current is limited by thermalgeneration from the field-free p-type absorbing volume, described morefully below.

Diode dark current is due to minority carrier electrons from the p-typeregion (depending of course on implementation—see, e.g., the example ofFIGS. 2A/2B, discussed below) plus minority carrier holes from then-type region. These flows of minority carriers manifest themselves ascurrent in the output circuit of the diode. In one example, the presentinnovations minimize minority carrier hole flow, leaving the electronminority carrier flow to dominate.

FIGS. 2A and 2B depict one example consistent with the presentinnovations. FIG. 2A shows a simplified device structure, and FIG. 2Bshows a corresponding energy band diagram. In FIG. 2A, the device 200 isshown in side view. N+ region 202 is layered atop a relatively lightlydoped N region 204, which is itself located atop a highly doped P region206. N+ region 202 and N region 204 both have wide bandgaps relative tothe P region 206. In preferred embodiments, the present innovations areembodied using a heterojunction, which provides differing bandgapsbetween the various regions where the composition differs. The N+ region202 has a contact 208 by which voltage is applied to the device relativeto the p contact 214. In preferred embodiments, the heterojunctiondevice is operated in reverse bias.

The interface between N region 204 and P region 206 by definitioncontains band discontinuities. In the case of HgCdTe of differentcompositions on either side of the interface, the larger discontinuityoccurs in the conduction band. This potentially abrupt discontinuity, orbarrier to minority carrier flow from the P region, is preferablysoftened by a degree of compositional grading (e.g., in region 204A)such that the resulting field associated with the graded bandgap can beovercome by a modest reverse bias voltage V applied to the diode, asdepicted in FIG. 2B. In this manner, photo-generated electrons in the Pregion absorber can migrate from the P region into the N region (oranother region left of the P region, as shown in the diagram) of the SADdiode for detection. Graded region 204A can vary in size, but ispreferably limited to the N region (in this example).

FIG. 2B shows one example energy band diagram consistent with theinnovative structure shown in FIG. 2A. In this example, P region 206 isadjacent to N region 204, followed by N+ region 202. P region 206 actsas the photon absorbing region, where light (preferably of IRwavelength) is incident. The incident photons have enough energy tocause electrons to enter the conduction band. This of course depends onthe bandgap of the P region, which is selected depending on thewavelength of radiation the device is designed to detect. When photonsof the proper energy strike the P region 206, the excited electrons movetoward the depletion region from within an electron diffusion length ofthe junction, preferably located substantially (or more preferably,completely) in the N region 204. N region 204 preferably includes aregion of compositional grading 204A which softens the slope of thediscontinuity of the conduction band threshold between the P and Nregions.

In this example, the solid line 210 depicts the bandgap under zero bias,and the dashed line 212 depicts the bandgap under reverse bias. Asshown, barrier region 204A still offers a barrier to minority carriersfrom the P region 206 when the device is under zero bias. However, whenthe device is under reverse bias, the barrier is preferably entirelyflattened, allowing the carriers excited by the photons to move from theP region 206 into the N region 204, and then to continue that movementas determined by the electric fields of the device. For example, anelectrode contacted to material in the N region 204 or the N+ region 202could measure the current through the device to thereby detect photonsof a proper wavelength striking the P region 206.

The following paragraphs describe example implementations consistentwith preferred embodiments of the present innovations. None of thestated requirements are intended to limit the scope of the presentinnovations, but are only intended to serve as an example of how thepresent innovations can be implemented.

The separate absorption and detection (SAD) diode is a diodearchitecture consisting of a p-type infrared (IR) semiconductorabsorbing region coupled to an adjacent n-type diode region of higherbandgap than the absorber. The p-region is heavily doped relative to then-region such that the depletion region of the diode is confinedentirely to the n-type higher bandgap region of the device. Fortemperatures such that n_(a)>n_(i), where n_(i) is the intrinsic carrierconcentration of the absorbing p-region, the overall dark current of awell-made diode is dominated by generation through Shockley-Read (S-R)centers in the depletion region of the diode. Thus, the wide bandgapnature of the n-region will enable a significant reduction in thelimiting dark current of the SAD diode. The bandgap of the n-region issimply chosen to be large enough that the dark current of the diode islimited by thermal generation from the field-free p-type absorbingvolume. The diode architecture and band diagram are shown in FIG. 2. Theunderlines of the N+ and N regions in FIG. 2( a) indicate that theseregions are wide bandgap relative to the P region. The interface betweenthe N and P regions by definition contains band discontinuities. In thecase of HgCdTe of different compositions on either side of the interfacethe larger discontinuity occurs in the conduction band. This potentiallyabrupt discontinuity, or barrier to minority carrier flow from the Pregion, needs to be softened by a degree of compositional grading suchthat the resulting field associated with the graded bandgap can beovercome by a modest reverse bias voltage V applied to the diode, asindicated in FIG. 2( b). In this manner photo-generated electrons in theP region absorber can be detected by the N region of the SAD diode.

The primary benefit of the SAD diode lies in the reduction of thermallygenerated dark current in the device relative to the standard IRabsorbing homojunction, or the conventional double layer heterojunction(DLHJ), which is wide bandgap P+ on N, where N is the absorber. Theprimary dark current components per unit volume in a well-made one-sidedHgCdTe homojunction photodiode are as shown in FIG. 3, and consist ofdiffusion currents from either side of the junction, together withthermal generation through S-R centers in the depletion region of thediode. Tunneling currents can also play a role but normally just serveto limit the doping concentration and bias voltage that can be utilizedin the depletion region.

The diffusion current has two components associated with Auger andShockley-Read (S-R) generation. On the N-side, in the non-equilibriumenvironment of a reverse-biased diode, the Auger1 component isqn/2τ_(Ai1), and the S-R component qn_(i)2/nτ_(SRn), where τ_(Ai1) isthe Auger1 lifetime, n the donor concentration, and τ_(SRn) the S-Rlifetime of minority carrier holes in N-side material. Likewise for theP-side, the Auger7 component is qp/2τ_(Ai7), and the S-R componentqn_(i) ²/pτ_(SRp). In material with a density of S-R centers that islower than any relevant doping concentration,τ_(SRn)=τ_(po)[(n+n_(i))/n]+τ_(no)n_(i)/n, andτ_(SRp)=τ_(no)[(p+n_(i))/p]+τ_(po)n_(i)/p, where τ_(no)=1/γ_(n)N_(r),and τ_(po)=1/γ_(p)N_(r). γ_(n) and γ_(p) are the recombinationcoefficients for electrons and holes into the N_(r) neutral centerslocated at the intrinsic energy level in the bandgap of thesemiconductor. For neutral centers at the intrinsic energy levelτ_(no)˜τ_(po)˜τ. Thus the S-R diffusion current is given by, on theN-side qn_(i) ²/(n+2n_(i))τ, and the P-side by qn_(i) ²/(p+2n_(i))τ.Thus for temperatures at which n, p>n_(i) then the S-R diffusion currentvaries as 1/n, or 1/p. The Auger components on the other hand vary as n,or p. Thus on each side of the junction the diffusion current willminimize at a doping concentration that equalizes the two diffusioncomponents due to Auger and S-R. In the one-sided junction of FIG. 3 thedepletion region is devoid of majority carriers and lies entirely on theN-side. The thermal generation rate/unit volume through the S-R centersfor this majority carrier free region is qn_(i)/2τ. The relativemagnitudes of these various N- and P-side dark current components areshown in FIG. 4, for a representative case of 5 μm cutoff HgCdTe at140K. The model assumes a thickness of 3 μm for both N- and P-sides, anS-R lifetime, τ=10⁻⁴ s, and an absence of vacancies from the P-volume.Literature values for τ_(Ai1), and τ_(Ai7) are assumed withτ_(Ai7)=60τ_(Ai1). Tunneling considerations are seen to limit the donordensity <7×10¹⁵ cm⁻³ for V=0.3V.

The limitations of the homojunction diode are clearly exposed in FIG. 4.The P-side dark current minimizes at ˜3×10⁻⁸ A/cm², whereas the N-sideis completely dominated by the depletion current component to values ˜1to 3×10⁻⁶ A/cm² at usable doping concentrations. This property of thehomojunction is unavoidable, as the optimized dark current generationrate per unit volume in the diffusion regions is qn_(i)²/(N_(maj)+2n_(i))τ, but in the depletion region is qn_(i)/2τ. Foroperating temperatures where N_(maj)>>n_(i) then the depletion currentgeneration rate per unit volume will always be larger than the diffusioncurrent by the ratio N_(maj)/n_(i), which can be very large, regardlessof the value of the S-R lifetime.

The SAD diode eliminates this dark current limitation of thehomojunction (and also the P+/N DLHJ) by increasing the bandgap E_(g2)on the N-side to a value which renders the dark current from the N-sidesmaller than the optimized dark current from the P-side.

This aspect of the SAD diode will be true for any absorbing cutoffwavelength for which n_(i), at the system operating temperature, is muchless than the doping concentration in the absorbing P-layer. FIG. 8shows the dependence of n_(i) versus cutoff wavelength for a range ofoperating temperatures of HgCdTe. A realistic doping concentration forthe p absorbing region is obtained by optimizing it for approximateequality of the Auger7 and S-R diffusion currents at the specificoperating temperature. For reasonable S-R lifetimes in HgCdTe thisoptimized doping concentration is typically 10¹⁵ to 10¹⁶ cm⁻³. Thus arealistic value of useful operating temperature for any cutoffwavelength can be obtained from FIG. 8 by assuming that n_(i) needs tobe <10¹⁴ cm⁻³. For example meaningful reductions in dark current for a10.5 μm SAD diode should be achievable at temperatures <100K, and for a5 μm SAD diode at temperatures <180K.

Similar arguments apply for an N+/P−/P SAD structure as shown in FIG. 9,and it is relevant to consider the dark current generated from thegraded bandgap transition region. This is obtained by integrating thedepletion current from incremental depletion regions Δx, assuming thatthe bandgap increases linearly with distance x as you traverse thetransition region of width d. If the bandgap increases from E_(g1) toE_(g2) then the transitional bandgap gradient, or effective electricfield, is given by F_(t)=[E_(g2)−E_(g1)]/d. The resulting dark currentfrom this transition region is J_(d)=[2kT/qF_(t)]n_(i)(E_(g1))/2τ. It isimperative that this dark current be less than the dark current from theabsorbing P region of the SAD diode, which gives a condition on therequired value of F_(t). It is also preferred that the bandgap E_(g2)again be chosen so that the dark current from the wide bandgap region isless than the P absorbing volume.

There is a limit on F_(t) in that it needs to be small enough that theelectric field required to overcome the minority carrier barrierassociated with the transition region does not generate tunnel breakdownin the wide gap region E_(g2). The tunnel breakdown condition in HgCdTefor a diode with a uniform applied electric field F is given byJ _(tunnel)=26.4FVexp(−1.062×10⁷ E _(g) ² /F)A/cm²,where F is the applied field in V/cm, V the applied voltage in V, andE_(g) the bandgap in eV. A uniform electric field is a reasonably goodapproximation for the low doped region of the SAD diode, assuming areasonably thin E_(g2) region (˜1 to 2 um). The dependence of J_(tunnel)on bandgap for different electric field values is shown in FIG. 10. Itis apparent that a field of 10⁴V/cm requires a value of E_(g)=0.2 eV toavoid breakdown.

The sequence for selection of required parameters for the successfuloperation of a SAD diode follows:

-   -   1. Select the desired cutoff wavelength, thus defining E_(g1).    -   2. The p-type doping of the absorbing region will optimize at        10¹⁵ to 10¹⁶ cm⁻³ for most cutoff wavelengths in state of the        art HgCdTe.    -   3. The doping of the E_(g2) region must be <<p-doping of the        absorber, such that a one-sided depletion region is obtained in        the SAD diode. A value as low as possible is desired, such as        ˜10¹⁴ cm⁻³, which is possible for HgCdTe. Such a value will        ensure that none of the applied voltage will generate a        depletion region in the E_(g1) absorber region.    -   4. Choose the operating temperature such that n_(i)<<p-doping,        say n_(i)<10¹⁴ cm⁻³.    -   5. The requirement on depletion current from the transition        region is that it be <diffusion current from the E_(g1) absorber        region. If we assume that diffusion current from the absorber is        S-R limited then the requirement on F_(t) is that        [kT/qF_(t)]qn_(i)/τ<qn_(i) ²t/pτ, which simplifies to        F_(t)>(kT/q)(p/n_(i))/t, where t is the absorber thickness which        will be ˜1/α, where α is the absorption coefficient.    -   6. The required F_(t) value determines the applied field (F),        which in turn determines the value minimum of E_(g2) for the        wide gap detecting region.    -   7. This value of E_(g2) must also result in a depletion current        from the wide gap region that is <diffusion current from the        p-absorber, and also does not exhibit tunnel breakdown under an        applied field F>F_(t).    -   8. The values of F_(t), E_(g1) and E_(g2) determine the width of        the transition region required.    -   9. The value of F_(t), together with the thickness of the E_(g2)        region, determine the required applied voltage on the SAD diode.        Regarding point 7 above: The difference in bandgaps between the        absorber and detector regions should preferably be large enough        that the depletion current from the wide gap region be less than        the diffusion current from the p-absorber region. Following the        discussion above, this implies that        qn _(i)(E _(g2))W/(2τ)<qn _(i)(E _(g1))² t/(pτ)        is the S-R lifetime, W the thickness of the wide where τ gap        region, and t is the thickness of the absorber. This then        implies the condition that        n _(i)(E _(g2))<n _(i)(E _(g1))²[2t/W]/p.        For p˜2e15 cm⁻³ and [2t/W]˜5, this condition translates into        E_(g2)>1.7E_(g1), and is true for all meaningful operating        temperatures.        This condition holds approximately true as long as you don't        have to worry about tunnel current, which becomes a problem for        E_(g2)<0.1 eV. This means that you can use the approximate        condition that E_(g2)>1.7E_(g1), as long as E_(g2) is >0.1 eV,        i.e. as long as E_(g1)>0.059 eV. For E_(g1)<0.059 eV then we        need to make E_(g2) equal to or greater than 0.1 eV.

Two practical requirements that must be borne in mind are that

-   -   1. The transition region thickness will be limited by the vapor        phase growth technique used to generate the hetero-structure.        MBE utilizes a typical growth temperature of ˜175 C and as such        provides a minimum of inter-diffusion between regions.        Transition regions >500 A wide should be possible with this        technique. MOCVD growth on the other hand occurs at ˜350 C and        inter-diffusion is a significant issue. Transition regions <1 to        2000 A may not be possible with this vapor phase technique.    -   2. The thickness of the E_(g2) region needs to be at least ˜2 um        to allow for the formation of an n+ region by ion etching or ion        implantation. This would result in a diode depletion width of at        least ˜1 um.        MWIR (5 um cutoff) at 140K (for N+/P−/P SAD Diode Structure)

The following discussion is given in the context of a SAD diode withN+/P−/P structure, though the present innovations are not limited toonly that embodiment. Select p doping=5×10¹⁵ cm³; p−=10¹⁴ cm⁻³; t=6 um,thickness of E_(g2) region W=1 um.

From FIG. 8 n_(i)˜10¹³ cm⁻³ so p/n_(i)˜500. The requirement on F_(t) is>(kT/q)(p/n_(i))/t, so we have F_(t)>10⁴V/cm, which easily satisfies thetunneling criterion established for this field in FIG. 10 whichindicates that E_(g2)>0.2 eV. The bandgap E_(g2) is thus determined inthis case not by a tunneling requirement but by the dark currentconsideration that depletion current from E_(g2)<diffusion current fromE_(g1). Thus again assuming that E_(g1) is diffusion limited then werequire that qn_(i)(E_(g2))W/2τ<qn_(i)(E_(g1))²t/pτ, which givesn_(i)(E_(g2))<n_(i) ²(E_(g1))[t/W]/p, where W is the width of the E_(g2)region. Again from FIG. 8 we have n_(i)(E_(g1))=10¹³ cm⁻³, giving arequirement of n_(i)(E_(g2))<1.2×10¹¹ cm³, which corresponds to abandgap E_(g2)=0.355 eV. Thus the thickness of the transition region isgiven by [E_(g2)−E_(g1)]/F_(t)=0.105/104=1050 A.

Similar calculations for LWIR (10 um at 77K) and SWIR (2.5 um at 240K)are included in Table 1. for the SAD N+/P−/P diode, with t=6 um and W=1um.

TABLE 1 SAD Parameters for Various Absorber Cutoffs p E_(g1) p- E_(g2)E_(g1) F_(t) E_(g2) d (cm⁻³) (cm⁻³) (eV) T (K) V/cm (eV) (A) Bias (V) MW(5 um) 5 × 10¹⁵ 10¹⁴ 0.25 140 10⁴ 0.355 1050 1.0 LW (10 um) 5 × 10¹⁵10¹⁴ 0.124 77 7.7 × 10³ 0.191 870 0.77 SW (2.5 um) 10¹⁶ 10¹⁴ 0.496 3004.2 × 10³ 0.653 3738 0.42

Similar arguments apply for the N+/N−/P SAD diode concept, as well asother embodiments of the present innovations.

The present innovations can be embodied in a number of structures, withvariations in the doping and materials used, operating conditions,thicknesses, and other parameters. Examples presented herein are onlyintended to be illustrative, and do not imply limitations to theinnovative concepts as presented. Some further example implementationsare presented below.

Though the present innovations are useful in a wide array ofimplementations and situations, preferred embodiments use theseinnovations in the context of an array of IR detector cells.

One example implementation is depicted in FIGS. 5A and 5B. FIG. 5A showsa side view of a photodiode consistent with implementing an embodimentof the present innovations, while FIG. 5B shows an energy band diagramcorresponding to that architecture.

FIG. 5A shows a back-side illuminated unit cell of a photodiode 500 witha mesa N+/N−/P architecture, preferably using the material HgCdTe ofvarying constituencies (described more fully below) in a heterostructureoperated at non-equilibrium, in reverse-bias. In this example, photons514 strike a backside surface (substrate 502) of diode 500. P region 504is fabricated atop substrate 502. P region, in this example, preferablyserves as the absorbing region, where photons 514 excite carriers(preferably minority carriers) into the conduction band 516 (see FIG.5B). N region 506 is formed atop P region 504. In preferred embodiments,P region 504 is heavily doped relative to N region 506 such that thedepletion region of the interface is confined substantially (preferablyentirely) to N region 506. In this example, P region 504 has energybandgap of E_(g1), and a doping concentration of 10¹⁶ cm⁻³. N region 506has bandgap E_(g2) and doping concentration of 10¹⁴ cm⁻³. In preferredembodiments, E_(g2)>E_(g1). In HgCdTe systems, there is a conductionband discontinuity between the P and N regions. In preferred embodimentsof the present innovations, this discontinuity is softened bycompositional grading of the HgCdTe constituencies, forming a slopeddiscontinuity 524 in N region 506 adjacent to P region 504. Under zerobias 520, the discontinuity presents a barrier to minority carrier flowfrom P region 504 into N region 506. However, under reverse bias 522,the barrier 524 is reduced or disappears entirely, allowing minoritycarrier flow.

Atop N region 506 is formed a more heavily doped N+ region 508 thatallows majority carrier contact to the device, where voltage 510 isapplied. In this example, ground 512 is also contacted to P region 504.The device is preferably operated in reverse bias, so that when photons514 impinge on the device, they excite minority carriers of P region 504into conduction band 516, which can move toward N regions 506, 508 wherethey can be detected. This detection is preferably indicated by a changein current of the device. Device 500 is, in one example embodiment, bumpbonded to a read-out integrated circuit (ROIC), which contains circuitelements useful in receiving output of the diode 500 to serve as aphoton detector. Detection of the photons can occur, in one preferredembodiment, as a detection of minority carrier current flowing throughthe diode into an output preamp. This current can then be integrated ona capacitor for a finite time and measured. Other ways of measuring theoutput can also be used.

FIGS. 6A and 6B show another example implementation consistent with thepresent innovations. This is another example of a backside illuminateddevice 600, where photons 614 are received at a substrate 602. Thesubstrate has formed thereon a P region 604, which is followed by a P−region 606 and an N+ region 608 that forms a diode region in 606 andserves to apply voltage 610 to the device. The diode has a depletionregion 605 as shown. P region 604 is also grounded 612 in this example.P region 604 is preferably heavily doped relative to P− region 606,whereas P− region 606 has a wide bandgap relative to P region 604. Inthis example, P region 604 has doping of 10¹⁶ cm⁻³, while P− region 606has doping of 10¹⁴ cm⁻³.

FIG. 6B shows the energy diagram for this example structure, includingvalence band 618 and conduction band 616. In this example, when underzero bias 620, the device shows a barrier 624 in conduction band 616 tominority carriers from P region 604 to P− region 606. However, when thedevice is put under reverse bias 622 the barrier 624 disappears, whichallows carriers from P region 604 to move into P− region 606 and N+region 608 where they can be detected, as described above. Barrier 624is best implemented using compositional grading of a HgCdTe material, inpreferred embodiments.

One advantage to the P/P−/N+ diode architecture is that a planarstructure can be made which has advantages over mesas due to passivationissues.

FIGS. 7A and 7B show another embodiment of the present innovations,including structure (FIG. 7A) and energy diagram (FIG. 7B). The device700 includes ROIC 702 which is contacted by contact metal 710, whichitself is fabricated through various layers, including P region 704, P−region 706, and N+ region 708. Similar to some previously describedembodiments, P region 704 is preferably doped such that the depletionregion occurs substantially (preferably entirely) in P− region 706,which has a larger bandgap than P region 704. Photons 714 occur at thetop of the device, and interact with P− region 706 and P region 704.Contact metal 710 is preferably isolated electrically from P and P−regions by insulator 712. Contact metal 710 can be formed, for example,as a via. P region 704 can be grounded remotely (not shown in figure).

FIG. 7B shows the effect of reverse bias on the device. At zero bias720, barrier 724 resists minority carrier flow from P region 704 to P−region 706. However, under reverse bias 722, barrier 724 is reduced oreliminated, allowing carrier flow out of P region 704.

FIG. 8 shows a graph of intrinsic carrier concentration v. HgCdTe cutoffwavelengths at various temperatures. Embodiments of the presentinnovations, such as the SAD diode of FIG. 5 (for example), will reducedark current relative to a homojunction diode if the net dopingconcentration, N_(a), in the p-type absorbing layer is much greater thanthe intrinsic carrier concentration n_(i) at the chosen operatingtemperature. Thus, for N_(a)=5×10¹⁵ cm⁻³ at 180K reduced dark currentwould be obtained for HgCdTe cutoff wavelengths less than 7 microns(assuming that N_(a)>5 n_(i)). At 300K, cutoff wavelengths need to beless than 3.5 microns. Thus, as we increase operating temperature, theadvantage of the SAD diode is primarily seen at the shorter cutoffwavelengths.

FIG. 9 shows diagrams of bandgaps consistent with preferred embodimentsof the present innovations, including the top diagram for zero appliedvoltage and the bottom which shows the system under reverse bias. Inthis example, region 902 is, for example, a heavily doped P region,regions 904 and 906 are P− region, and region 904 is the depletionregion, preferably located entirely in the P− region 906. In the lowerdiagram, showing the reverse bias case, the potential barrier is flat,allowing minority carrier flow from P region 902 to N region 906 wheredetection can occur. This diagram also shows contribution to darkcurrent from the depletion region. The intrinsic carrier concentrationis a function of horizontal position (x) due to linear bandgap variationwith x in the transition region. As shown in the figure, the depletioncurrent from a region Δx is J_(dep)(x)=qn_(i)(x)Δx/2τ_(sr).

FIG. 10 shows a log plot of tunnel current and bandgap. The bandgap ofthe absorbing material must of course be of a size such that photons ofthe proper energy can excite carriers across the bandgap. The plot ofFIG. 10 shows different bandgap plots and tunnel currents for differentapplied electric fields. These values can be used to determineconstraints on the device. For example, if it is determined that atunnel current of less than 10⁻⁹ A/cm² is desirable, and if the photonsto be detected require a bandgap no larger than 0.1 eV, then the appliedelectric field is constrained (in this example) to somewhere near 3×10³V/cm.

FIG. 11 shows profiles of doping, charge, and electric field for twodifferent embodiments of the present innovations.

Separate Absorption and Detection (SAD) Diode for 2-Color Operation

The SAD diode described above can also be operated as a single contactsequential (or pseudo-simultaneous) 2-color detector by using differentlayers of the device (which preferably have different bandgaps) asdetectors for different colors of light or wavelengths of radiation,preferably IR radiation. For example, in one embodiment (using an N+/N/Pstructure similar to that shown in FIG. 2) a two-color detector isachieved by using the wider bandgap N-region 204 as a detector of thesecondary color, and the P-side 206 absorber as a detector of theprimary color. The bandgaps of the two absorbing regions of thisembodiment should be chosen according to the particular wavelengths thatare to be detected, such that the incident photons have enough energy togenerate minority carriers to the conduction and valence bands (acrossthe bandgap) for that particular region. Thus, in this example, sincethe N region 204 has a wider bandgap than P region 206, the N regionwould be used to detect photons of higher energy, while the P regionwould be used to detect photons of a lower energy, where the photons ofthe higher energy are referred to as the secondary color and the lowerenergy photons are referred to as the primary color.

In this example, the secondary color will be detected at essentiallyzero bias across the SAD diode, and the primary color detected by theP-type absorber will not register in the diode output circuit due to theinterfacial barrier 204A to minority carrier electrons that existsbetween the heterojunction regions of the SAD architecture. The P-sidesignal is read out by the application of a modest to high reverse biasto the SAD diode as shown in FIG. 2. This bias is sufficient to overcomethe interfacial barrier, resulting in both colors being read out in theoutput circuit of the SAD diode. The primary color is preferablyobtained by differencing the output signals of the SAD diode at low andhigh reverse biases, though other means of distinguishing the signalscan also be implemented within the context of the present innovations.

FIG. 12 shows an example diode architecture 1200 consistent with apreferred embodiment of the present innovations. This example deviceincludes N+ region 1202, P− region 1204, and P region 1206 attached toROIC 1208. Contact 1212 is made at the top of the device and connects toROIC 1208 through a via or trench through the N+, P−, and P layers.Contact 1210 is separated from the layers of the diode by insulator 1212shown on both sides of this cut-away view.

Photons 1214 are preferably incident on the device such that they reachP− region 1204 before reaching P region 1206. In this example, P− region1204 has a bandgap which is larger than that of P region 1206. Thismeans, in this example, that P− region 1204 is the secondary colordetector, while P region 1206 is the primary color detector.

This example is depicted as a high density vertically integratedphotodiode (HDVIP), and the top-side contact 1210 to the P− region 1204can be made in the standard HDVIP manner. The N+ region 1202 isgenerated, for example, by a suitable ion implant. This architecture issuitable as a single unit cell configuration for an array of detectors.A backside contact (not shown) can be implemented, for example, aroundthe periphery of the focal plane array, or can be included within theunit cell, and is preferably P+ in nature. If within the unit cell it ispreferably small in volume or of a wider bandgap than the absorbingregion of the photodiode. It can also be implemented remote to the unitcell.

FIG. 13 shows another example embodiment for the bump-bonded version ofthe two-color concept. In this example, photons 1302 are incident on agrowth substrate 1304 which has layered atop it an N+ region 1306, a P−region 1308 (with bandgap E_(g2)), a P region 1310 (with bandgap E_(g1)which is narrower than E_(g2)), and a P+ region 1312 (with bandgapE_(g3)). The wider bandgap composition E_(g2) can be grown first,followed by the narrower bandgap E_(g1), resulting in the multilayerstructure shown, where the E_(g2) composition includes the N+ contactlayer, which can be of the E_(g2) composition or of even larger bandgap.The narrower bandgap E_(g1) layer is preferably capped with a P+ contactlayer of wider bandgap E_(g3), so as to avoid dark current from thiscontact. Mesas 1314 can be then etched into the P− region to provideindividual diodes for subsequent bump bonding and sequential address for2-color operation.

FIG. 14 shows another example implementation for the 2-color concept. Inthis example, the P+ E_(g3) layer 1412 is grown first, followed by the PE_(g1) layer 1410, and then P− E_(g2) layer 1408. The P− layer 1408 canthen be subjected to an ion implant to create the N+ layer 1406 and thestructure is preferably transferred to a sacrificial IR transparentsubstrate 1414 as shown. Growth substrate 1404 is then removed, and thisis followed by mesa 1416 etching and bump bonding in preferredembodiments.

FIG. 15 shows a timing diagram for one embodiment of the 2-color deviceoperation, consistent with a preferred embodiment. FIG. 15 shows twocharts with parallel time axes. The top chart shows diode appliedvoltage, where the lower chart shows charge on integration capacitor.During the low bias condition charge is collected on the integrationcapacitor due to flux current from the wider bandgap region of the diodestructure. This integrated charge is read out after τ_(int1) and thecapacitor node is reset whilst the reverse voltage on the diode is resetto a large value. The capacitor then integrates charge due to fluxcurrent from both the wider bandgap and narrower bandgap regions of thediode. This integrated charge is read out after τ_(int2) and the noderesets again whilst the diode is reset to the low reverse bias voltage.The cycle then repeats. The second color flux is obtained by subtractionof the two readings weighted by the appropriate integration times.Possible errors due to non-simultaneity can be reduced by using morerapid addressing of the integration capacitor to achievepseudo-simultaneous operation. In preferred embodiments, a lower limiton the integration times used is set by the quiescent charge thataccumulates in the narrower bandgap region during τ_(int1). This isdetermined by the minority carrier lifetime in that region, τ_(min). Forminimum corruption of narrow bandgap signals τ_(int2)>>τ_(min), which istypically a few microseconds.

Integration of the signals can take different forms, including even oruneven accumulation during the two integration times. For example, ifthe two wavelengths of detected light are of different fluxes, theweaker flux light period can be accumulated for a longer period. Othervariations in this procedure can also be implemented within the contextof these innovations.

In another embodiment, the quiescent charge can be avoided by a suitabletime lapse between the application of the larger reverse bias and thestore of the integration period.

One primary advantage of the single contact mode of operation of the SADdiode is the simplicity of the structure, and that there is no region ofdiode forward bias within the focal plane unit cell, which is verydifferent to the present state of the art in single contactheterojunction detection, which typically utilizes back-to-back diodeswith a floating base. One issue with the back-to-back diode architectureis that the forward-biased wider bandgap diode acts as an efficientemitter of bandgap radiation, resulting in crosstalk into thereverse-biased narrower bandgap diode. There is no such interactionbetween the diode regions in the SAD diode.

Most or all of the commentary concerning the SAD diode concept describedabove is appropriate for the 2-color SAD concept. Some differences withthe 2-color diode are that:

-   -   1. The incident photon flux must strike the shorter of the two        cutoff wavelength regions (bandgap E_(g2)) first so that the        longer cutoff wavelength can pass through without absorption and        be detected by the region of bandgap E_(g1).    -   2. The region of bandgap Eg2 must be of sufficient thickness so        as to absorb a significant fraction of the incident photon flux        with energies >E_(g2). The absorption coefficient of HgCdTe as a        function of photon energy E is given by        α=6×10⁴[E(E−E_(g))}^(1/2). The thickness W of region E_(g2) is        preferably ˜1/α, which is typically on the order of a wavelength        of the IR radiation.    -   3. The bandgap Eg2 is determined by the second color photon        energy, and not by dark current considerations.    -   4. The operating temperature of the 2-color SAD diode is not        limited by the condition N_(maj)>n_(i), which is purely a        condition on dark current minimization at lower temperatures.

FIG. 16 shows a charge trans-impedance amplifier input circuit 1600consistent with implementing a preferred embodiment of the presentinnovations. This example depicts one example of the ROIC's inputcircuit stage, which can be used to read the output of the photodiodeembodiments described above. The photodiode 1604, under an applied biasvoltage V, incrementally passes charge in response to photon flux 1602.This charge is integrated on capacitor C_(int) 1606, which can be resetat the end of each integration time. Op amp 1608 accordingly provides anoutput voltage V_(signal) at terminal 1610, which represents the chargeseen on the diode 1604 during the current integration time. This analogstage topology is merely exemplary, and can of course be modified withvarious additional components for amplification or noise reduction, ordifferent circuit topologies can be used.

Modifications and Variations

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given.

Note that the single-contact structure of the present application can beused in combination with, or separately from, the teachings in thepriority application which relate to reduced dark current. Theconsiderations related to dark current reduction are very extensivelydiscussed in the present application, but these considerations are NOTnecessary for practice of most of the claimed inventions. In particular,two-color operation can be achieved if the height of the potentialbarrier is only a few kT, without satisfying the more stringentconditions which fully optimize dark current.

Though multiple architectures have been described (e.g., the P/N/N+, andthe P/P−/N+), the present innovations can be implemented with differentmaterial arrangements and relative doping concentrations than have beenused as examples. Further, the materials and compositions given hereinas examples are not the only possible implementations consistent withthese innovative ideas. For example, HgCdTe has been used as exemplary,but other materials that conform to the innovations herein described canbe used as well.

The means of arranging the individual diodes into an imager or detectorare also not intended to limit application of these innovations. Anyarchitecture or arrangement that incorporates the presently disclosedinnovations is within contemplation of the present inventions.

The fabrication process used to create a device consistent with theseinnovations can also vary. For example, graded compositions can beobtained in a variety of ways, including but not limited to MBE(molecular beam epitaxy) and metal organic chemical vapor deposition(MOCVD), for example.

The relative positions of various layers, contacts, dopants, and otheraspects of the fabricated device can vary yet remain within the scope ofthe present innovations.

The specific use of the present innovations mentioned herein are alsoonly examples, and are not intended to limit the scope of the presentinnovations. For example, though IR detection is a contemplated example,other uses for the structures or methods or systems disclosed hereincould be found without deviating from the present innovative concepts.

Additional general background, which helps to show variations andimplementations, may be found in the following publications, all ofwhich are hereby incorporated by reference:

-   “Microchip Fabrication,” Peter Van Zant, McGraw Hill, 2000;-   “HgCdTe heterojunctions,” Peter R. Bratt, J. Vac. Sci. Technol. A    1(3) July-September 1983;-   “Common Anion Heterojunctions: CdTe—CdHgTe,” Migliorato and White,    Solid-State Electronics Vol. 26, No. 1, pp. 65-69, 1983;-   “Mercury Cadmium Telluride for High Operating Temperature Infrared    Detectors,” Ashokan, et al; Arias, J M; Zandian, M; Williams, G M;    Blazejewski, E R; Dewames, R E; Pasko, J G (1991):-   HgCdTe dual-band infrared photodiodes grown by molecular-beam    epitaxy (technical note), J. Appl. Phys. 70(8, 15 October),    4620-4622;-   Blazejewsli, E R; Arias, J M; Williams, G M; McLevige, W; Zandian,    M; Pasko, J (1992): Bias-switchable dual-band HgCdTe infrared    photodetector. J. Vac. Sci. Technol. B 10(4, July-August),    1626-1632;-   Tennant, W E; Thomas, M; Kozlowski, L J; McLevige, W V; Edwall, D D;    Zandian, M; Spariosu, K; Hildebrandt, G; Gil, V; Ely, P (2001): A    Novel Simultaneous Unipolar Multispectral Integrated Technology    (SUMIT) Approach for HgCdTe IR Detectors and Focal Plane Arrays. J.    Electron. Mater. 30(6), 590-594;-   Almeida, L A; Thomas, M; Larsen, W; Spariosu, K; Edwall, D D;    Benson, J D; Mason, W; Stoltz, A J; Dinan, J H (2002): Development    and Fabrication of Two-Color Mid- and Short-Wavelength Infrared    Simultaneous Unipolar Multispectral Integrated Technology    Focal-Plane Arrays. J. Electron. Mater. 31(7), 669-676;-   Rajavel, R; Jamba, D; Johnson, S (1997): Molecular Beam Epitaxial    Growth and Performance of Integrated Two-Color HgCdTe Detectors    Operating in the Mid-Wave Infrared Band. J. Electron. Mater. 26(6,    01 June), 476;-   Mitra, P; Barnes, S; Musicant, B (1997): MOCVD of Bandgap-Engineered    HgCdTe p-n-N-P Dual-Band Infrared Detector Arrays. J. Electron.    Mater. 26(6, 01 June), 482;-   Johnson, S; Johnson, J; Gorwitz, M (2000): HgCdZnTe Quaternary    Materials for Lattice-Matched Two-Color Detectors. J. Electron.    Mater. 29(6, 01 June), 680;-   Smith, E P G; Pham, L T; Venzor, G M; Norton, E M; Newton, M D;    Goetz, P M; Randall, V K; Gallagher, A M; Pierce, G K; Patten, E A;    Coussa, R A; Kosai, K; Radford, W A; Giegerich, L M; Edwards, J M;    Johnson, S M; Baur, S T; Roth, J A; Nosho, B; De Lyon, T J; Jensen,    J E; Longshore, R E (2004): HgCdTe Focal Plane Array for Dual Color    Mid- and Long-Wavelength Infrared Detection, J. Electron Mater. 33,    June, 509-516;-   Coussa, R A; Gallagher, A M; Kosai, K; Pham, L T; Pierce, G K;    Smith, E P; Venzor, G M; De Lyon, T J; Jensen, J E; Nosho, B Z;    Roth, J A; Waterman, J R (2004): Spectral Crosstalk by Radiative    Recombination in Sequential-Mode, Dual Mid-Wavelength Infrared Band    HgCdTe Detectors. J. Electron. Mater. 33(6, 1 June), 517-525;-   Buell, A A; Pham, L T; Newton, M D; Venzor, G M; Norton, E M; Smith,    E P; Varesi, J B; Harper, V B; Johnson, S M; Coussa, R A; De Leon,    T; Roth, J A; Jensen, J E (2004): Physical Structure of    Molecular-Beam Epitaxy Growth Defects in HgCdTe and Their Impact on    Two-Color Detector Performance. J. Electron. Mater. 33(6, 1 June),    662-666;-   Ballet, P; Noel, F; Pottier, F; Plissard, S; Zanatta, J P; Baylet,    J; Gravrand, O; De Bomiol, E; Martin, S; Castelein, P; Chamonal, J    P; Million, A; Destefanis, G (2004): Dual-Band Infrared Detectors    Made on High-Quality HgCdTe Epilayers Grown by Molecular Beam    Epitaxy on CdZnTe or CdTe/Ge Substrates. J. Electron. Mater. 33(6, 1    June), 667-672;-   Baylet, J; Gravrand, O; Laffosse, E; Vergnaud, C; Ballerand, S;    Aventurier, B; Deplanche, J C; Ballet, P; Castelein, P; Chamonal, J    P; Million, A; Destefanis, G (2004): Study of the Pixel-Pitch    Reduction for HgCdTe Infrared Dual-Band Detectors. J. Electron.    Mater. 33(6, 1 June), 690-700;-   Sood, A K; Egerton, J E; Puri, Y R; Bellotti, E; D'Orsogna, D;    Becker, L; Balcerak, R; Freyvogel, K; Richwine, R (2005): Design and    Development of Multicolor MWIR/LWIR and LWIR/VLWIR Detector    Arrays. J. Electron. Mater. 34(6, June), 909-912;-   D. K. Blanks, J. D. Beck, M. A. Kinch and L. Colombo, J. Vac. Sci.    Technol. A6, 2760 (1988);-   U.S. Pat. No. 5,189,297;-   U.S. Pat. No. 5,279,974;-   U.S. Pat. No. 6,049,116;-   U.S. Pat. No. 6,603,184; and-   U.S. Pat. No. 4,961,098.

None of the description in the present application should be read asimplying that any particular element, step, or function is an essentialelement which must be included in the claim scope: THE SCOPE OF PATENTEDSUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none ofthese claims are intended to invoke paragraph six of 35 USC section 112unless the exact words “means for” are followed by a participle.

The claims as filed are intended to be as comprehensive as possible, andNO subject matter is intentionally relinquished, dedicated, orabandoned.

1. A multicolor photodetection structure, comprising: a monolithicsemiconductor heterostructure defining a photodiode, the photodiodecomprising first and second photoabsorption regions of two differentbandgaps and an N+ region formed on the first photoabsorption region,the first photoabsorption region having a first conductivity type andthe second photoabsorption region having a second conductivity type; afirst contact electrically connected to the N+ region; a backsidecontact electrically connected to the second photoabsorption region; andcircuitry electrically connected to the N+contact region via the firstcontact and electrically connected to the second photoabsorption regionvia the backside contact, wherein the circuitry: in a first mode,biases, via only the first contact and the backside contact, both saidphotoabsorption regions to collect photocarriers therefrom, withoutforward bias on either; and in a second mode, biases, via only the firstcontact and the backside contact, only one of said photoabsorptionregions to collect photocarriers therefrom, without forward bias oneither.
 2. The structure of claim 1, wherein photons strike the firstphotoabsorption region and thereafter strike the second photoabsorptionregion; and wherein higher energy photons are absorbed in the firstregion and lower energy photons are absorbed in the second region. 3.The structure of claim 2, wherein the first region is at least as thickas the wavelength of the lowest energy photons absorbed by themulticolor photodetection structure.
 4. The structure of claim 2,wherein the circuitry toggles between the first and second modes.
 5. Thestructure of claim 1, wherein the first photoabsorption region is an N−region, and the second photoabsorption region is a P region.
 6. Thestructure of claim 5, wherein the depletion region of the photodiode isconfined substantially in the first photoabsorption region.
 7. Thestructure of claim 1, wherein the first photoabsorption region is a P−region, and the second photoabsorption region is a P region.
 8. Amonolithic multicolor infrared photodetection structure, comprising: amonolithic semiconductor heterostructure defining a photodiode, thephotodiode comprising first and second photoabsorption regions of twodifferent bandgaps, and an N+ region formed on the first photoabsorptionregion, the first photoabsorption region having a first conductivitytype and the second photoabsorption region having a second conductivitytype; control circuitry which is bonded to the monolithicheterostructure and electrically connected to the N+ region and thesecond photoabsorption region, but constructed from a differentsemiconductor material, and which operates the photodiode in reversebias via only the electrical connections to the N+ region and the secondphotoabsorption region to obtain different signals from the photodiodecorresponding to the two different bandgaps, wherein said electricalconnections to the N+ region and the second photoabsorption regioncomprises a first contact and a backside contact; wherein the controlcircuitry: in a first mode, reverse biases, via only the first contactand the backside contact, both said photoabsorption regions to collectphotocarriers therefrom; and in a second mode, reverse biases, via onlythe first contact and the backside contact, only one of saidphotoabsorption regions to collect photocarriers therefrom.
 9. Thestructure of claim 8, wherein the photoabsorption regions are positionedso that incoming photons enter the first photoabsorption region beforethe second photoabsorption region, and wherein the first photoabsorptionregion has a wider bandgap than the second photoabsorption region. 10.The structure of claim 8, wherein the first photoabsorption region is anN region, and the second photoabsorption region is a P region.
 11. Thestructure of claim 8, wherein the first photoabsorption region is a P−region, and the second photoabsorption region is a P region.